1. Field of the Invention
This invention relates to a field instrument for directly measuring the permeability of rock in situ.
2. Description of the Related Art
Permeability is an important formation characteristic that indicates how fast oil or gas may flow from an oil or gas bearing formation. Moreover, permeability is the only elementary rock property directly related to fluid flow, and permeability cannot be accurately estimated from other rock properties. Accurate measurement of rock permeability is difficult and requires the measurement or determination of fluid flow responsive to a pressure differential.
Two methods have commonly been used to measure rock permeability. One method obtains permeability of a formation by monitoring changes in the pressure of a borehole as fluid is pumped out. This method is useful for measuring the permeability of subsurface rocks in situ. Its disadvantage is that only an average permeability is obtained, leaving detailed permeability structures within the formation unrevealed. A second commonly used method requires that a plug be drilled from the formation. The plug is placed in a rubber sleeved core-plug holder known as a "Hassler-sleeve" and is sealed in place by a confining pressure applied to the outside of the sleeve. A pressure difference is applied across the length of the plug to induce fluid flow through the plug. The rate of flow and the pressure difference are measured, and the permeability is computed by applying a mathematical formula known as Darcy's law. This approach is described, for example, in Freemann et al. U.S. Pat. No. 4,555,934 and Jones et al. U.S. Pat. No. 4,573,342. Although this approach allows for more detailed studies of formation permeability, it is time consuming, it does not measure permeability in situ, and it is destructive to samples.
An instrument known as a mini-permeameter has been used for non-destructive measurement of rock permeability. As described in Eijpe and Weber, "Minipermeameters for Consolidated Rock and Unconsolidated Sand," AAPG Bull., Vol. 55, No. 2 (February 1971) p. 307-309, the instrument consists essentially of a narrow tube which is pressed with a controlled force against a flat, clean rock surface. Air is forced from the tube through the pores and flows out around the tube. A rubber ring at the tube's tip prevents leakage between the tip and the rock surface. A constant pressure drop is applied, and the air-flow rate is measured with a rotameter unit. The permeability of the sample is derived from this flow rate and the applied pressure. For field work, the pressurized air is supplied by a cylinder of compressed air or a small compressor. As further described in Goggin et al., "Patterns of Permeability in Eolian Deposits," SPE/DOI Fifth Symposium on Enhanced Oil Recovery, Tulsa, Okla. (April 20-23), 1986, the injection tip is pressed against an outcrop surface after the top 1/4 cm of each measurement site is chipped away. The gas flow rate is estimated by a series of rotameters which are selected to accurately cover a wide range of possible rates. An estimate of the permeability may be obtained by calibrating the minipermeameter flow rate for various core plug samples of known permeability.
Operation of the minipermeameter in the field has been a laborious task prone to frequent error. To obtain the permeability of a rock formation in situ, measurements are taken at a large number of test sites at spaced intervals in an array or grid. Although the sensing of permeability occurs rapidly at each test site, the overall process requires a considerably greater amount of time for leveling the rotameters, monitoring the flow rate, adjusting pressure, and recording the field measurements. Also, bulky cylinders or an air compressor is needed to supply the relatively large amount of air required for sensing permeability at a large number of test sites.